file_is_displayable_image example

function wp_generate_attachment_metadata( $attachment_id, $file ) {
    $attachment = get_post( $attachment_id );

    $metadata  = array();
    $support   = false;
    $mime_type = get_post_mime_type( $attachment );

    if ( preg_match( '!^image/!', $mime_type ) && file_is_displayable_image( $file ) ) {
        // Make thumbnails and other intermediate sizes.         $metadata = wp_create_image_subsizes( $file, $attachment_id );
    } elseif ( wp_attachment_is( 'video', $attachment ) ) {
        $metadata = wp_read_video_metadata( $file );
        $support  = current_theme_supports( 'post-thumbnails', 'attachment:video' ) || post_type_supports( 'attachment:video', 'thumbnail' );
    } elseif ( wp_attachment_is( 'audio', $attachment ) ) {
        $metadata = wp_read_audio_metadata( $file );
        $support  = current_theme_supports( 'post-thumbnails', 'attachment:audio' ) || post_type_supports( 'attachment:audio', 'thumbnail' );
    }

    /* * wp_read_video_metadata() and wp_read_audio_metadata() return `false` * if the attachment does not exist in the local filesystem, * so make sure to convert the value to an array. */